(a) Field of the Invention:
The present invention relates to an illumination optical system for an endoscope.
(b) Description of the Prior Art:
Conventionally known illumination optical systems for endoscope generally have the composition shown in FIG. 1. Speaking concretely, an illumination lens 3 is arranged in front of, i.e., on the object side of, the light exiting surface of a light guide 1 so that the light coming out of the light guide 1 is divergent in a wide range. However, the above-mentioned type of illumination optical systems have a drawback described below. That is to say, since the rays emerging at a wide angle from the exiting surface of the light guide are lost due to the scattering on the peripherical inside surface of the illumination lens 3 and directed toward the center of the visual field and the light guide due to the total reflection on the peripherical inside surface of the illumination lens 3, light distribution is made insufficient at the marginal portion of the visual field, thereby making it impossible to provide uniform illumination within a wide range.
Known as a conventional illumination optical system contrived to correct the above-mentioned drawback is the illumination optical system disclosed by Japanese Unexamined Utility Model Application No. 17071/57. This conventional illumination optical system comprises an illumination lens which has a composite surface consisting of a spherical central surface and a peripherical conical surface on the side of the light guide. However, this illumination optical system is incapable of freely controlling light distribution and produces ununiform illumination since the illumination lens has the composite surface consisting of the spherical surface and the conical surface.